Hitherto, as lithographic printing plates, so-called Presensitized Plates have been used, wherein a light-sensitive composition is applied onto an aluminum plate to form a light-sensitive layer. In the above-described aluminum plate, a rough surface is formed by a process suitably selected from a mechanical roughening process such as ball graining, wire graining, brush graining, liquid honing, etc., an electrochemical roughening process such as electrolytic graining, etc., a chemical roughening process and a combination of two or more of them, by which a satin finish is obtained on the surface. Then, it is etched, if necessary, with an aqueous solution of acid or alkali and subjected to anodic oxidation treatment. Thereafter, it is subjected, if necessary, to a treatment for providing a hydrophilic property to produce a support for a lithographic printing plate. On the treated surface, a light-sensitive layer is provided to produce the presensitized plate. This presensitized plate is then subjected to exposure to light, development, retouching, gumming-up, etc. to produce a printing plate, which is then placed on a printing apparatus to carry out printing.
Although there are many processes for treating the surface of an aluminum plate, known processes have various faults. For instance, in the case of ball graining, there are problems in that high skills are required for selection of the kind (material) or the size of balls, control of water in carrying out abrasion, determination of abrasion time and evaluation of the finished surface due to a batch processing, and productivity is very inferior. In the case of wire graining, the roughness of the resulting surface of the aluminum plate is non-uniform. In case of brush graining, high roughness is not obtained on the treated surface, and scattering is easily formed on the coarse face by the wear of the abrasion brush used. Further, there are problems that the surface of aluminum is scratched by the strong friction between the brush and the abrasive so as to form many sharp projections like molding projections, by which the light-sensitive layer to be removed by development of the presensitized plate remains to cause stains on the plate face, or scratches are easily formed on the surface by rubbing of the treated surface (rough surface) in the case of handling the aluminum plate. In the case of liquid honing, since a slurry liquid containing a fine abrasive powder dispersed in the liquid is sprayed at a high rate by compressed air, the fine abrasive powder easily sticks to the surface of aluminum, to thereby form projections; further in this process, there are problems in that the roughness of the surface cannot be sufficiently increased because the impulsive force of the slurry liquid against the surface of aluminum is small and that the setting nozzle wears significantly because the slurry liquid is jetted at a high rate. In the case of electro-chemical roughening, it is necessary to carry out minute control of the electrolysis condition in order to keep the treated surface at a constant roughness, and the consumption of electric power is rather large; moreover, disposal of waste liquor containing Al ions accumulated in the electrolyte requires great expense. In the case of chemical roughening, the time required for treatment is relatively long and, consequently, it is not suitable for mass production. Further, great expense is required for disposal of waste liquor as in the case of the electro-chemical process.
In order to attempt to overcome some of the above-described disadvantages associated with each graining process, an improved process comprising a combination of the brush graining or wire graining and the electrolytic graining has been proposed, as disclosed in U.S. Pat. No. 2,344,510 and Japanese Patent Application (OPI) No. 123204/78 (the term "OPI" as used herein refers to a "published unexamined patent application"), and British Pat. Nos. 1,582,620 and 2,047,274. Further, an improved process comprising a combination of the liquid honing and the electrolytic graining is disclosed in European Patent Application (OPI) No. 131926. According to such combined process, however, when the brush graining is adopted as a first step, i.e., a mechanical graining step, stains are apt to be formed in the non-image areas during printing, and when the wire graining is adopted as the first step, the printing plate has poor printing durability.
When the liquid honing is adopted as a first step, stains are not so much formed in the non-image areas during printing, but the printing plate is poor in printing durability. Thus, there is need to further improve the quality of a printing plate so as to meet the desire for better printing plates.